The present invention relates generally to the field of electrochemical cells. More particularly, this invention pertains to lithium batteries in which the cathode comprises an electroactive sulfur-containing material and the cathode current collector comprises a conductive support and a crosslinked polymeric conductive primer layer.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As consumer demand for, and reliance upon, portable and hand-held electronic devices such as mobile telephones, portable computers, pagers and palm pilots has grown, so has the need for portable power supplies, such as rechargeable batteries, with long cycle life, rapid recharge capacity, and high energy density become more important. There has been considerable interest in recent years in developing high energy density primary and secondary batteries with alkali-metal anode materials, and, in particular, anodes based on lithium.
One component of a battery, especially a rechargeable battery, which is important for long cycle life, rapid charge capacity, and high energy density, is the current collector. In rechargeable lithium batteries, for example, current collectors have typically been constructed of nickel or aluminum. Aluminum is generally preferred due to lower cost and lower density. Unfortunately, current collectors constructed from aluminum exhibit a relatively high interfacial impedance associated with the presence of an oxide layer on the surface, which, in turn, results in a loss of energy and reduced power. Such losses are generally attributed to the fact that the aluminum surface includes a native oxide layer which (a) acts as an insulator, increasing interfacial impedance, and thus severely limits electrical conductivity, and (b) greatly hinders adhesion of electrochemically active electrode materials which are to be applied to the surface of the aluminum current collector during electrode fabrication.
Two approaches have been presented to improve aluminum current collector functioning. One approach emphasizes primer layers on the aluminum with improved adhesion, and a second approach emphasizes reduction of interfacial resistance at the aluminum interface. In the first approach polymers containing carboxylic acid functionality have been found to provide strong adhesion to aluminum metal surfaces. For example, in U.S. Pat. No. 5,827,615, Touhsaent et al. show that adhesion of polyvinyl alcohol polymers to aluminum films is improved by the use of carboxylic acid containing polymers in the formulation, such as olefin-maleic acid copolymers. Chassar et al., in U.S. Pat. No. 6,069,221, report that a carboxylic acid function in polymer formulations improves adhesion to metals, particularly aluminum. Similarly, in U.S. Pat. Nos. 5,441,830 and 5,464,707 to Moulton et al., the adhesion-promoting properties of carboxylic acid functionality is described in an electrically conducting primer layer material. In U.S. Pat. Nos. 5,399,447 and 5,520,850, to Chaloner-Gill et al., is described an adhesion promoting layer containing conducting material and a polymer, such as a polyacrylic acid, and a lithium salt to reduce acidity and reactivity to a lithium anode. An example of a conductive primer layer for a current collector from a crosslinked polymer from a crosslinking reaction of a polymer having pendant carboxyl groups, such as ethylene/acrylic acid polymers, with a multifunctional crosslinking agent, is described in U.S. Pat. No. 5,478,676 to Turi et al.
Improved current collector performance by reduction of interfacial resistance has been described, for example, in U.S. Pat. Nos. 5,578,396, 5,591,544, and 5,588,971 to Fauteux et al., by freeing the surface of an aluminum current collector of oxide, etching the surface with a carboxylic acid material to improve adhesion, and providing a primer, such as graphite, to prevent re-growth of the oxide layer. Although carboxylic acid polymers may provide excellent adhesion to aluminum current collectors, problems of corrosion of the aluminum by cell components may still exist.
Another approach to reduce the corrosion of aluminum current collectors is described in U.S. Pat. No. 5,518,839, to Olsen, in which a layer of a corrosion resistant metal, such as nickel, copper, chromium, titanium, or mixtures thereof, is applied to an etched aluminum current collector surface. Such an approach, however, adds an additional process step and adds weight and cost to the cell.
It is thus an object of the present invention to provide a current collector for electrochemical cells and method of manufacturing same, wherein the current collector has a substantially reduced interfacial impedance and substantially increased adhesive capabilities.
The cathode current collector of the present invention for use in an electrochemical cell comprises: (a) a conductive support, and (b) a conductive primer layer overlying the conductive support, the primer layer comprising from about 25 to 70% by weight of a crosslinked polymeric material formed from a reaction of a polyvinyl acetal and a crosslinking agent, and about 30 to 75% by weight of a conductive filler; and wherein the cell comprises: (i) an anode comprising lithium, and (ii) a cathode comprising an electroactive sulfur-containing material.
The polyvinyl acetal is preferably selected from the group consisting of polyvinyl butyral and polyvinyl formal. In one embodiment, the crosslinking agent comprises phenolic resins. The conductive filler is preferably selected from the group consisting of carbon black, graphites, activated carbon fibers, non-activated carbon nanofibers, metal flakes, metal powders, and electrically conductive polymers. The conductive support is preferably selected from the group consisting of aluminum foil and aluminized plastic films.
In one embodiment, the weight ratio of the polyvinyl acetal to the crosslinking agent in the crosslinked polymeric material is from 4:1 to 2:3. Preferably, the thickness of the conductive primer layer is from about 0.2 to 5 microns.
In one embodiment, the electroactive sulfur-containing material comprises elemental sulfur. In another embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises a polysulfide moiety of the formula, Sm, wherein m is an integer equal to or greater than 3. In yet another embodiment, the electroactive sulfur-containing material comprises a sulfur-containing organic polymer.
Another aspect of the present invention pertains to methods of preparing a cathode current collector of an electrochemical cell, wherein the current collector, as described herein, is formed by the steps of: (a) coating onto a conductive support a liquid mixture comprising a polyvinyl acetal, a crosslinking agent, a conductive filler, and a liquid medium; (b) drying and crosslinking the coating formed in step (a) to yield the current collector. The drying and crosslinking step (b) is preferably performed at a temperature of from about 70xc2x0 C. to about 180xc2x0 C.
In one embodiment, the liquid medium comprises one or more organic solvents.